S-antigen, a protein located in retinas of all vertebrates, is of importance for its capacity to induce a disease, experimental autoimmune uveitis (EAU) in animals immunized with microgram amounts of this antigen. Further, accumulating data suggest that S-antigen may be involved in certain pathogenic autoimmune processes in the human eye. The purpose of this project is to purify and characterize this antigen, in order to better understand its immunopathogenic effect. Using the technology of "high performance liquid chromatography" (HPLC), a procedure was devised for purification of S-antigen to homogeneity. Purified preparations will be useful for further studies concerning the identity and biological function of S-antigen. Other experiments showed that S-antigen is resistant to boiling and detergent treatment, which increase the digestability of the molecule by enzymes. Digestion with a bacterial proteolytic enzyme was found to degrade S-antigen to fragments of small molecular sizes. Rats immunized with a pool of these fragments developed EAU, thus showing that these components retain the capacity to cause EAU.